1. Field of the Invention
The present invention relates to a refrigeration circuit containing a refrigerant oil mixture. More specifically, this invention relates to a control for energizing a heater provided to raise the temperature of the lubricating oil for a compressor in a refrigeration circuit and in particular to a control for selectively energizing the heater in response to the operating conditions of the refrigeration system.
2. DESCRIPTION OF THE PRIOR ART
Air conditioners, refrigerators, heat pumps and other devices utilizing a refrigeration circuit produce a controlled heat transfer by the selective evaporation and condensation of refrigerant under varying temperature and pressure conditions. A compressor may be utilized to increase the temperature and pressure of gaseous refrigerant and to circulate that refrigerant through the refrigeration circuit. Typically, oil for lubrication of the compressor components is contained with the refrigerant within the refrigeration circuit.
It is well known, under certain conditions, that most refrigerants and oil used as a lubricant for the compressor of a refrigeration unit are freely miscible. During normal operation of the refrigeration circuit, because of the operating pressures and temperatures, the oil will be maintained in the sump of the compressor and will be substantially free of refrigerant. However, on shutdown when the circuit reaches ambient temperature, and the pressure equalizes within the circuit, refrigerant vapor and oil in the sump of the compressor will mix to form a substantially homogenous solution. This phenomenon becomes increasingly evident as the ambient temperature decreases.
Upon startup of the compressor, the oil sump which is usually a part of the crankcase of the compressor drops to suction pressure and the compressor mechanism may agitate the mixture of lubricating oil and refrigerant. A combination of the drop in suction pressure and possible mechanical agitation causes the refrigerant in solution with the oil to attempt to return to its gaseous state. Since the refrigerant at shutdown is in a substantially homogenous solution the flashing of the admixed liquid refrigerant to vapor may carry therewith a substantial amount of the oil charge and may even result in the entire solution turning into a foam.
Foaming of the oil will materially increase the amount of oil carried over into the refrigerant discharge line. Foaming may become so severe that all of the oil is pumped out of the sump. Not only will this leave the compressor without lubrication, which may produce excessive bearing wear and bearing failure in a very short period of operation, but there is also a possibility that noncompressible slugs of liquid refrigerant and oil will enter the compressor's cylinders and cause serious damage to the compressor in the form of broken valves and pistons and bent or broken connecting rods and shafts.
To avoid the problem of crankcase oil dilution with refrigerant crankcase heaters are generally employed. The heater may be an electrical resistance element and may be installed directly in the sump of the compressor in direct contact with the oil or may be wrapped about the outer surface of the compressor casing in heat transfer relation with the oil stored in the sump. The energization of the heater will maintain the lubricant at a satisfactory temperature above ambient, for example 40.degree. F. to 60.degree. F. above ambient. At this temperature, only a small amount of refrigerant will be absorbed by the oil charge.
Heretofore, it has been a practice within the industry to either maintain the heater energized at all times regardless of the operation of the refrigeration unit or the temperature of the ambient. Alternatively, it has been the practice to render the heater inoperable when a refrigeration unit is functioning and to energize the heater when the refrigeration unit has been shut down. Another practice has been to de-energize the crankcase heater when the ambient temperature is above a certain level or when the refrigeration unit is operating.
When a refrigeration system is operated, heat energy is stored in the compressor. This heat energy acts to maintain the refrigerant and oil separate for a certain time period after the compressor operation has been discontinued. The length of this time period has been found to be a function of the length of the period of operation of the compressor. Hence, in order to save energy it has been found to be efficient to operate the crankcase heater when the compressor is not operated and after the time period when the heat stored in the compressor during operation has been dissipated.